THE VASCULAR CRYPTOGAMS 59 



begins as a single cell. It undergoes several cell- 

 divisions (see Fig. 28, F, c). The cells thus produced 

 round themselves off and become the mother-cells of the 

 spores. In many Ferns there are sixteen or more 

 mother-cells in each sporangium, but in the Male Fern 

 there are usually not quite so many. 



The spore mother-cells are spherical; as the sporangium 

 has grown more rapidly than they have, they do not fill 

 the whole interior, but float freely in a half-liquid mass 

 derived from the disorganised tapetal cells. Each 

 mother-cell now divides twice so as to form four cells, 

 each of which has at first the shape of a quadrant of a 

 sphere (see Fig. 28, D). These four daughter-cells are 

 the spores. As they ripen they become kidney-shaped, 

 the convex side corresponding to the free outer surface of 

 the mother-cell, while the concave edge of each spore 

 represents the line of junction with its sister-cells. The 

 spore membrane becomes much thickened, and consists 

 of two layers, the outer of which is strongly cuticularised, 

 and assumes a dark-brown colour. We have now seen 

 how the myriads of microscopic dust-like spores which 

 we find on the back of a Fern-frond are produced. It 

 remains for us to learn how they are scattered. 



It is the annulus which causes the sporangium to 

 open ; dehiscence takes place when the wall of the ripe 

 sporangium has begun to dry up. The cells of the 

 annulus lose water, and consequently contract, the thin 

 outer walls of the cells becoming concave instead of 

 convex (see Fig. 28, C). The final result of this con- 

 traction is that the whole annulus violently straightens 

 itself, and in so doing necessarily tears the sporangium 

 open, the rupture taking place across the broad thin- 

 walled cells at the end of the annulus (see Fig. 28, (7, 



